A note on computing uniform risk spectra from intensity data on earthquake occurrences

Reliability of uniform risk Fourier and response spectra, derived from Modified Mercalli Intentsity data, on earthquake occurrences has been investigated by comparing these spectra with the corresponding spectra based on magnitude data. By treating the site intensity from ab earthquake, with given maximum intensity and epicentral distance, as a random variable, a Bayesian probability approach is suggested to compute the intensity spectra, in the present study. Examples of comparison of the intensity and the magnitude spectra for two typical sites in the highly seismic northeast Indian region have shown very good consistency and agreement between the two spectra. Thus, in the proposed approach, intensity data can be used to get quite reliable design spectra for regions where the history of instrumentally recorded data is not sufficient for engineering seismic risk analysis.